The purpose of this study is to quantitate the relative roles of genetics and environment in the etiology of blood pressure levels in newborn twins. Elevated blood pressure has been clearly identified as a major risk factor of coronary heart disease, stroke and congestive failure in adults. Studies in adults indicate that there are strong family similarities for blood pressure levels. These findings were duplicated several years ago in children age 2 to 14 years. At the University of Miami, it has recently been shown that the one-month-old infant has blood pressure levels more like his brothers and sisters than like all other one-month-old infants. In addition, correlation between the blood pressure at birth, and that taken in the same infant at one month of age is significant. These findings support the hypothesis that prenatal determinants, probably genetic, play a role in the establishment of blood pressure levels. A study of twins, provides the best-known model for quantitation of the relative roles of genetics and environment in the etiology of blood pressure levels. Understanding of etiology may provide a rational basis for predicting those persons who are likely to have elevated blood pressure.